Several new ribonucleoside 3',5'-cyclic phosphates and related nucleosides have been synthesized. These include: 8-selenoadenosine 3',5'-cyclic phosphate, 8-methylselenoadenosine 3',5'-cyclic phosphate, 8-ethylselenoadenosine 3',5'-cyclic phosphate, 8-benzylselenoadenosine 3',5'-cyclic phosphate, 8-selenoadenosine, 8-methylselenoadenosine, 8-ethylselenoadenosine, 8-benzylselenoadenosine and 6-ethylselenoinosine. The cell cultures, enzymatic hydrolysis of 8-substituted seleno cyclic nucleotides, were investigated. It was found that these seleno cyclic nucleotides were more resistant to phosphodiesterase hydrolysis than cAMP. It also indicated that cyclic nucleotides are more active than the corresponding nucleosides in mouse leukemia cell cultures, although none of these compounds inhibits cell growth at very low concentrations. It is planned during the next year that most of the proposed research projects will be completed, such as 3',5'-cyclic phosphate of 6-selenoguanine ribonucleoside, 3',5'-cyclic phosphate of 6-thioguanine ribonucleoside and their corresponding seleno, sulfur deoxy (alpha, beta) purine ribotides will be synthesized. In addition to synthesizing the above new cyclic nucleotides, the 8-substituted selenocyclic nucleotides will also be studied, if possible. It is also anticipated that all the new compounds will be evaluated in several biological systems, cell cultures, enzyme preparations, immunosuppressive activity, transplantable tumor systems.